1. Field of the Invention
The invention relates to a printing head for ink jet printers, comprising a plurality of tubular nozzle ducts, each of which has associated with it a drive member for the droplet-wise ejection of ink droplets at appropriate instants, the ink being applied thereto via a common distribution duct and individually associated damping members, the nozzle ducts with their drive members being accommodated in a first block and the common distribution duct being accommodated in a second block, said blocks being preferably made of a plastic material and being interconnected.
2. Description of the Prior Art
A printing head of this kind is known, for example, from DE-OS No. 26 59 398 and comprises several ducts and drive members in order to form an ink jet printing mechanism for the printing of characters in the form of a matrix of ink droplets. Such printing heads essentially consist of two plastics blocks. In a first plastics block the tubular nozzle ducts and their drive members are encapsulated. The ducts are encapsulated as straight ducts in a fan-like pattern and their front ends open into a separate nozzle plate in which the nozzles are arranged to be parallel with respect to one another. Moreover, each duct is formed so that at the area of the cylindrical drive members it is cylindrical with a comparatively large diameter. The geometrical arrangement of the nozzles in the nozzle plate is such that the distance between the nozzles satisfies the requirements imposed by the matrix-wise composition of the characters to be printed. For example, when the character has a height of 3.2 mm, the center-to-center distance of the nozzles amounts to 0.53 mm in the case of six nozzles. In order to satisfy this requirement, the nozzle ducts must be conical in the zone between the drive members and the nozzles. This means a higher expenditure as regards work and materials. The second plastics block contains a bowl-shaped recess in which the fluid is introduced. The two plastics blocks are interconnected at their edges. Between the plastics blocks there is arranged a further plate in which there are provided damping ducts (chokes) whose cross-section is smaller than the cross-section of the nozzle ducts and which are arranged parallel with respect to one another.
The known compact units impose very high requirements as regards their manufacture. In spite of the high manufacturing precision of the individual parts, mutual displacement in the transitional zones between the pressure duct and nozzle element and/or between the plate for the damping of the fluid and the duct during assembly cannot be precluded. This fact may have a significant adverse effect on the emission of droplets and may even cause failure of the printing head.
It is difficult to fill the known system with fluid without gas bubbles being trapped at the transitional zones. Any mutual displacement between the pressure duct and the nozzle element increases the risk of inclusion of gas bubbles, not only when the fluid is introduced for the first time, but also during operation because, for example, the fluid can flow back into the interior of the system due to shocks and air bubbles are trapped at critical areas as the fluid is accelerated. Any displacement between the pressure channel and the nozzle element, moreover, will in any case affect the transmission of pressure and cause pressure losses.
Furthermore, a gap is always present between the blocks with the nozzle ducts and the nozzle plate. This gap also influences the ejection of droplets and, moreover, is not constant over a prolonged period of time. For example, it changes due to thermal or chemical influences of mechanical arching. These changes necessitate operation of the system with new, correspondingly modified drive pulses after each change. This implies not only additional electronic steps, but also permanent availability of service.
Similarly, at the area of the fluid supply it is not ensured that the fluid can be supplied without problems, i.e. without gas inclusions. The addition of the extra plate with the chokes even necessitates accurate positioning thereof at this area. This again represents a critical zone of the kind already described with reference to the nozzle plate. If the cross-section of the supply bores is even smaller than that of the nozzle ducts, which is favorable in view of ejection, the described problems occurring during the filling with fluid are even greater.